MjRocket

Riddle me this!

Why is it that while sitting parked in my hot garage with the AC running it blows nice and cold.

However once I start driving it gets significantly less cold...Any ideas as to what causes this?

I have good pressure, 225 psi on the high side and 35psi low side.

I just replaced out the relay in the control unit and everything else seems to be functional.

NC928S4

Recirc Flap under glove box is open and blowing in ambient air when driving. I had same problem.

I jammed a paint stirrer between the parcel shelf and the flap to keep it closed until I get around to replacing the flap diaphragm.

SeanR

Check your HVAC Actuators and ensure that they are working as they should. Sometimes the Blue/Black check valve is not working correctly and when you start driving, the vacuum tank loses vacuum, causing the actuators to improperly function.

I think I said that wrong. The vacuum tank isn't going to empty, but the check valve failing lets the intake pull vacuum from the HVAC area also instead of holding it.

Alan

The coolant heats up... likely your hot water valve isn't closing, quite possibly your comb flap (heater box bypass flap) also isn't working. Possibly also your mixing flaps don't operate correctly. Possibly your temperature sensor loop is broken.

However under all these cases the hot water valve must be open to get heat in the cabin. If you have temp at min and AC on you should not get heat.

This could also be due to a vacuum leak that allows water flow when engine vacuum drops at wider throttle opening (but closes sufficiently at idle).

You will need to determine exact symptoms to deduce the exact cause.

Alan

Alan

In a correctly operating system this isn't enough to make much difference - if it did you have some other problems going on.

Alan

MjRocket

I did forget to mention I have the heater valve zip tied shut, as I have no need for the heater and wanted maximum cooling.

dr bob

Too many heater valves do leak by. The heater hoses are double-wall with fabric reinforcement, so will tolerate a hose pincher for a test period. I think Roger has identified a specific Mercedes valve as a replacement. Otherwise I'd be catalog shopping for a more robust valve.

A vacuum leak anywhere in the HVAC system can cause your symptoms. At idle with lowest manifold pressure, the leakage might be small enough to maintain sufficient vacuum to hold actuators closed. With engine load, pressure increases (read: vacuum decreases) in the HVAC loop enough that the leak is enough to allow actuators to relax. For a quick check, T into the vacuum supply line to the solenoid rack in the center console. That system should be tight, and with no cruise control in use pressure should stay low regardless of engine load.

You can verify check-valve operation by blowing into it. Air passes one way but not the other. You should be able to blow from the system side towards the brake-booster side.

Bill Ball

I had the first problem Dr. Bob mentions - a heater valve that pullrd closed properly but had a slight internal leak so some hot coolant passed by it. The clue to this is that the temperature of the air coming through the vents once the car warms up with the AC OFF is higher than ambient. If the heater valve is not working at all the vent air temp will reach 140F or so. If the valve is just a bit incompetent, like mine, the vent air temp exceeds ambient by 10F or so. This will interfere with AC cooling to SOME extent once the engine coolant temp reaches running temps. In my case, replacing the heater valve brought the vent temp down to ambient with the AC OFF and provided more effective AC.

So, if your routine vent air temp does get above ambient once the engine warms up, you have a partially leaky heater valve. If not, look elsewhere.

Alan

You cannot have active heat in the cabin unless the heater core is hot (unless you have holes in the floor/firewall or other major issues).

You could also loose AC effectiveness for various reasons - most especially in this case if the condenser fan wasn't working or was insufficient for the heat load. Check the fan operation.

Experiment a little while monitoring vent temps, better if you go faster? how long does it take for you to lose effectiveness - it it related to coolant temp? or engine load?

Alan

MjRocket

Its pretty much when I come to a stop the air gets much cooler, and as I move it gets less cool (not warm but not nearly as cold) doesn't seem to have anything to do with engine temp and fans appear to be working well.

When i get home, pull into the garage and park after driving a fair amount around town it gets Cold again almost instantly.

AirtekHVAC

This is a possibility...low charge. As the rpm of the compressor increases, as you accelerate, the suction pressure drops. If you are low on refrigerant, the pressure drops enough for the compressor to drop off on the low pressure safety. Seen it a bazillion times.

MjRocket

Just topped off the system to be sure that wasn't the issue, pressures look good on the Guage and I even bypassed the low pressure switch just to double check...Made no diffrence.

MjRocket

An update I looked into this and sure enough the flap was only partially engaging, if I gave it a little nudge with my finger pushing it fully closed it would then stay until shut off.

So I did as NC928s4 suggested and braced it fully shut.

So far seems to help.

I need to go for a good drive and see for sure. Is this the only place outside air is brought in to the cabin?

Either way I know this means I have other issues going on as Alan stated, but im Headed to the Hell GTG tomorrow and hopefully this will give me cold AC for the trip.

OTR18WHEELER

It is because you are getting sleepy while running the engine in the garage with the door closed.

MjRocket

Very true brother!

I do get a little light headed while working in the garage from time to time.

Especially having no cats, its a killer! ...lol